Insect enemies or friends

On the planet earth around 530 plants are carnivorous, i.e. specially adapted to capture and digest insects and other invertebrates. They only represent almost 0.2 % of the total of 250,000 flowering plants, so far described. It is a small portion, but this adaptation to carnivorous life has permitted those plants to colonize soils poor in organic matter, often peat bogs. Such plants occur at all latitudes. Certain carnivorous plants are epiphytic, that is growing on other plants, like Nepenthes, and others live in mountains (Heliamphora, Brocchinia). We may mention here also carnivorous fungi, but they only catch small nematodes and not insects. Myrmecophilous or ant loving plants are in about the same number, but their number is still increasing, with new discoveries, while the carnivorous ones seem to have reached more or less their final number. Botanists have tried to find relationship between all the real carnivorous plants. Recent phylogenetic analyses of nucleotide sequences indicate that stereotyped trap forms, leading to carnivorous habit, have evolved independently in different lineages of angiosperms (Albert et al, 1992).

Most of such plants show strange adaptations for insect capture and digestion (bright colours, nectar glands, digestive glands). They all live in selected biotopes, as mentioned earlier. Heliamphora species are found only on Venezuela mountains (the tepuys), as the protocarnivorous Brocchinia, and the truly carnivorous Dionaea muscipula, "one of the most wonderful plants in the world" (Darwin), is restricted to a small endangered area, wetland planted with pine trees, between North and South Carolina in the USA. Droseraceae family contains, however, also two ubiquitous genera,

Drosera and Utricularia. The rapid closure of the Venus flytrap (Dionaea muscipula) leaf in about 100 ms is one of the fastest movements in the plant kingdom (Forterre et al, 2005). The fast closure of the trap results from a snap-buckling instability controlled by the plant.

The carnivorous aspect of those plants is not at all exaggerated, and we know cases of gluttony. When these plants rot, the plant stomach digests itself by an excess secretion of pepsins. Genlisea, an aquatic plant, is a real living stomach in itself, very sophisticated, with only an anus missing, reducing similarity with our digestive system. It is the eeltrap of Darwin.

Carnivorous plants not only attract insects to eat them, but also to pollinate their flowers. In the latter case evidently they don't catch them. This is the case of a certain species of Nepenthes from Borneo, of which the flowers are pollinated by insects, and it maintains ants in their hollow petioles. The ants feed with impunity on the plant's extrafloral nectaries. The plant captures insects through their pitchers, but spares ants and the pollinating insects. It is a rare case of carnivory coupled with myrmecophily. This Nepenthes bicalcarata has been recently studied by an Australian team, after the old researches by Beccari in 1884 (Jolivet, 1887; Holldobler and Wilson, 1990). The Australians confirmed the old observations. In Nepenthes, the ant guests are protected by their behaviour and by their physical adaptations to the leaf trap and the surrounding slippery mucus. Several insects, like mosquitoes breeding inside the pitchers of carnivorous plants, are resistant to plant enzymes, particularly when the contents are diluted. Spiders and other predators fish with impunity the preys of Paepelanthus bromeloides (Eriocaul-aceae), a protocarnivorous plant, in Serra do Cipo, Brazil. Several predators, as some Hemiptera, live on the leaves of Roridula, among the glutinous hairs, without any harm. The digestive urns become diluted rapidly, when they open, and they quickly turn into phytotelmata, as in Nepenthes and other pitcher plants. Predators of the preys, caught by carnivorous plants are very diverse, from spiders to bugs and even beetles, and there are also slugs in Spain for one species of Pinguicula (Zamora and Gomez, 1996). Kleptopar-asitic (or stealing of the preys) behaviour is risk-free for the slugs in most carnivorous plants, because they are able to crawl on the leaves without being trapped. Recent researches on the trapping system among pitcher plants reveal a very sophisticated mechanism to attract preys and to induce the slipping of the prey.

Until recently carnivorous plants have been considered generally as insect killers, with only one objective, namely to improve their nitrogen supply, as they get a meagre natural diet in a poor soil. Now some of them are seen also as users of insects for pollination, offering, as a reward, the flower nectar.

Recently some new theories have been proposed (Juniper, Robins and Joel, 1989), mostly for pitcher plants, Sarraceniaceae and Bromeliaceae in America, Nepenthaceae in tropical Asia, the Seychelles and Madagascar, and Cephalotaceae in Australia, and some kind of mutualism between these plants and insects has been suggested. All are passive traps.

This theory could eventually be extended to other groups of carnivorous plants, but not to active traps, which actively catch their victims, while very few escape. Let us remind here that active traps are the ones as Drosera, Dionaea, which catch themselves the preys with their glutinous hairs or closing leaves, and passive traps are the ones such as pitchers plants which receive the preys which fall down by themselves.

Pitcher plants according to this theory seem to have developed attractive and efficient traps similar to those of many flowers to attract pollinators. Surely, insects do not visit those plants by mistake, and those plants do not resemble other flowers and are not able to attract the same insects. Also, the nectar from their pitchers, a real reward, is provided to insects visiting those plants. Of course, we cannot say that those traps do not resemble grossly the bright flowers of noncarnivorous plants, but they do not exactly copy any one of the latter in particular. They are only traps producing a sweet smell for their prospective preys.

On comparing, in the book on carnivorous plants Jolivet, 1987), ant plants (myrmecophilic) and carnivorous plants, PJ referred to the first relationship, that is in myrmecophily, as mutualism and the second as antagonism. Now, Joel (1988) suggests that carnivorous plants are also mutualists, at least the pitcher plants. He assumes that their relationships with the insect communities in their surroundings are not deceptive. According to him the insects benefit from the nectar, which is provided by the pitchers and which is nutritive and valuable in areas where other sources of nectar are rare, even absent. At the same time, the insects pay the plants thanks by offering a part of their population as prey to those plants growing on soils deficient in organic supply.

This opinion has been adopted in the book by Juniper, Robins and Joel (1989). The correctness of this view may be doubted, but when we observe attentively a pitcher plant, we can see that, after catching some prey, few insects are trapped, and many insects feed on nectar and fly away without being captured. Thus, at the insect population level, there seems to be an effective exchange of benefit between plants and insects.